RootStorageSvc+StorageSvc: Disable file aging for xAOD files.

POOL has a mechanism to automatically close files if they haven't been touched for a while. However, if the file contains xAOD objects, then we might have remembered pointers to branches in that file; closing it would leave dangling pointers. So, in RootTreeContainer, disable file aging if we were able to make an aux reader object.

In DbDatabaseObj::setAge, if the age is negative, disable further adjustments of it. This allows disabling the aging mechanism for a particular file by using setAge to force the age negative.